Interlaced vs non-Interlaced Questions

Discussion in 'Amateur Video Production' started by Kevin, Jun 17, 2006.

  1. Kevin

    Kevin Guest

    Excuse the long post but this has been bugging me for years now, and
    so far I never asked. I am creating some 352x480 MPEG-2 files for
    DVD. My DVD Authoring program will accept this size. Why I am using
    this size is a long story so I will skip that.

    My question is this. I am testing the files on my PC before buring
    the DVD. The pixel size, 352x480 is a "tall" ratio. Depending on
    which utility I convert the MPEG-2 with, the resulting video clip
    plays like a normal 4:3 video in WMplayer, and in other cases it plays
    very tall. My guess is that when it plays tall it is being played as
    progressive video and when it plays like a normal 4:3 video it is
    being played as interlaced.

    Using GSPOT to identify the MPEG's it always shows them as 352x480
    MPEG-2 streams, but does not tell me whether they are interl;aced or
    non-interlaced. None of the utilities I am using to convert the files
    has an option to interlace or non-interlace. I always though the
    video file itself can be either interlaced or non-interlaced,, because
    Premiere and even Photoshop both have a "de-interlace" feature. But
    now I am wondering if that is B.S. My questions are:

    1) does an MPEG-2 file itself have the capability to carry either
    interlaced or non-interlaced video?

    2) is there an internal flag or structure in the file that lets the
    software know whether the clip is interlaced or non-interlaced?

    3) is there any identification utility or any way that I can look
    into an mpeg-2 video clip and tell whether it is an interlaced video
    or a non-interlaced video? GSPOT does not tell me, and Premiere will
    not open MPEG-2 files.

    4) or are questions 1,2, and 3 alll bogus, and instead - is the file
    itself the same identical structure whether it is interlaced or
    non-interlaced? (this would explain why the conversion utilities
    never have an option to interlace or de-interlace).

    5) if #4 is the case, then is it the player that decides on the fly
    which way to display the video clip?
    Kevin, Jun 17, 2006
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  2. Kevin

    Jukka Aho Guest

    Not really. Pixel aspect ratio (and how to convert from one pixel aspect
    ratio to the other when displaying video on a system where the native
    pixel aspect ratio is not the same as in the video data) is one thing,
    interlacing is another. They're two unrelated things.
    Note that there are several types of "interlacing" to be considered.

    First of all, interlaced video (which consists of fields, not frames)
    can be stored in a format that nominally only allows progressive frames.
    How? By combining adjacent odd and even fields into "frames", and
    storing them as these combined frames instead. The storage format itself
    does not need to be aware of interlacing for this to work, but of course
    whoever _uses_ that data for anything should be, or otherwise he could
    accidentally display interlaced frames as if they were progressive,
    which won't look good.

    Then again, some video formats/codecs _are_ aware of interlacing, and
    offer special support for storing interlaced video data. These may range
    from fairly simple tricks (say, a simple flag in the data that denotes
    that the material is interlaced) to complex schemes where interlaced
    data is stored and encoded in a different way from progressive frames to
    enhance efficiency.
    MPEG-2 offers special flags for interlaced data, such as those which
    denote whether a decoded frame is top or bottom field dominant.

    In addition to this, MPEG-2 also supports different DCT coefficient
    scanning patterns for progressive and interlaced video data.
    GSPOT would appear to have indicators for the interlacing-related flags
    in the MPEG-2 streams:

    It is possible to store interlaced video in MPEG-2 format without
    flagging it so, though that would of course be stupid. (Then again, if
    there is even a remote possibility of something to be goofed up, it
    Jukka Aho, Jun 17, 2006
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  3. Kevin

    Ken Maltby Guest

    Pixel aspect ratio, Storage image size/shape, Display image, and
    "Resolution", are all concepts that can produce a great deal of
    confusion. For instance 720x480 is displayed as 4:3 or 16:9 and
    mathematically it is neither. To make it even more confusing you
    can, and DVDs often do, store a "widescreen" formatted image
    in MPEG with only the 720x480 storage image size.

    Providing an explanation that would completely and accurately
    address this issue, would be a long and painful effort. (Both to
    write and to read.)

    The short answer, as I see it, is :

    The DVD compliant 352x480 MPEG will be displayed properly
    by any DVD player. The 352x480 storage image contains the
    data to display a 720x480 image, with all the objects in their
    proper shape, circles won't become ovals.

    Depending on a number of factors, capturing a 4:3 NTSC TV
    analog signal to a 352x480 MPEG can provide a better
    Bits/Pixel ratio and more effective temporal compression than
    capturing at 720x480.

    Ken Maltby, Jun 17, 2006
  4. Kevin

    Jukka Aho Guest

    While that is true, the pedant in me would like to emphasize that
    352×480 is to be interpreted as a half-resolution variant of 704×480
    (which is another valid DVD MPEG-2 resolution for NTSC-compatible

    In other words, those 352 pixels, when decoded, should not be stretched
    (interpolated) over the full 720 pixels in the framebuffer, but only
    over the 704 centremost pixels, leaving the eight "missing" pixels on
    both side edges blank.
    Jukka Aho, Jun 17, 2006
  5. Kevin

    Ken Guest

    Thanks for the tip as to how to tell if a clip is interlaced or
    progressive, using the GSPOT flags.

    However, in my case, every time I create a 352x240 MPEG-2 file, GSPOT
    shows "Prog", yet when I try to open it in DVD Aithor, it refuses the
    file, saying it cannot open "Interlaced Video files". Strange.
    Ken, Jun 17, 2006
  6. Kevin

    Ken Maltby Guest

    Good point! But not too much of a problem if you can do a little
    "over projection" (sort of a reverse of underscan). Still that does
    waste the part of the frame buffer you mentioned.

    Ken Maltby, Jun 18, 2006
  7. Kevin

    Ken Maltby Guest

    I know you can't be referring to "TMPGEnc DVD Author" (TDA),
    as it like any other Author program can work with interlaced MPEG2
    as long as it is DVD compliant. There could be a problem trying to
    use interlaced video with a vertical component of only 240, and that
    could be the source of your problem. You normally need 480 lines to
    properly handle interlaced video conversion.

    You never mentioned how you are "creating" these files. Bye the way,
    running two threads related to the same issue/problem can make things
    somewhat confusing.

    Some advice; for DVDs; if you want to use 352x240 then make it CBR
    MPEG-1 and not interlaced. If you want to use interlaced video then
    make it MPEG-2 352x480 or larger. ( One of the main reasons MPEG-2
    was created was to handle interlaced video properly.)

    Ken Maltby, Jun 18, 2006
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